Hi Darren and welcome! I think the best thing to do by far is treat it as a clean sheet and dig it all over, removing every piece of root you see. Modern weedkillers only work when the plants are in full growth, so spraying now would be virtually useless - all of the perennial weeds have already gone dormant.
Rather than tackling the whole thing in one go, I'd do it in sections but doing each section well. That way you'll see results as you progress which is important to keep you motivated - it's all too easy to take on too much as most of experienced gardeners know only too well from their early days!
If you do it before the hard frosts set in, those will help to break up any big lumps of soil which you can leave on the top after digging it over. When spring comes, fork-in as much compost or well-rotted (it's important it is well rotted with no straw or soggy stuff) manure as you can lay yours hand on. General purpose multi-purpose compost is fine if you can't easily get farmyard manure or spent mushroom compost etc.
By doing it this way any plants or shrubs you plant will get off the a great start and have enough food for several years.